The present invention relates generally to image reading (or readable) devices and methods, and more particularly to a double-sided image readable device and method. A double-sided image readable device means a device that serves to read front and rear surfaces of a document simultaneously. The present invention is suitable for double-sided readable image scanners, copiers, facsimile machines, electrophotographic devices having flat bed unit and automatic document feed ("ADF") unit (or a sheet feed unit). A flat bed unit means a type that uses an optical system to read a document statically placed on a glass board, while an ADF unit means a type that automatically and mechanically feeds a document paper so as to read it using a static optical system.
As recent demands for effective use of resource have increased opportunities to use documents having images on its both sides, high-speed and high-resolution double-sided readable image scanners that may read the images on both document sides have been sought. An image scanner converts data indicative of lightness and density for each pixel color that has been obtained by scanning, into digital data as a result of comparison with white and black reference data. A double-sided readable image scanner typically includes flat bed and ADF units, a first optical system at the flat bed unit side, a second optical system at the ADF unit side, first and second white reference boards, and first black reference board. These white and black reference boards are fixed onto a housing. The first optical system reads one document surface, whereas the second optical system reads the other surface of the document. Each optical system includes a fluorescent lamp and CCD sensor, and reads the document by emitting lamplight onto the document and converting the reflection light into an electronic signal using the CCD sensor. The first white and black reference boards are arranged apart from a document feed path, and used as white and black references, respectively, for the first optical system. The second white reference board is located below the document feed path and used as white and black references for the second optical system.
In general, an image scanner corrects white and black reference data at the beginning of reading and at an appropriate intermission during a reading of a large amount of pages (for instance, every 50-page reading) in order to correct a change in quantity of lamp's light and the sensitivity dispersion of photosensitive pixels in the CCD sensor. According to the conventional correction method of white and black reference data, the first optical system, which is movable, moves to the first white and black reference boards and reads them while the lamp turns on. The second optical system, which is not movable, reads the second white reference board just below the feed path while the lamp turns on and sets the resultant value as a white reference. In addition, the second optical system reads the second white reference board while the lamp turns off and sets the resultant value as a black reference.
White and black reference values are generated as one line pixel data from the read white and black reference data, and stored in a memory. In reading the document, these white and black reference values are used for correction of image data (which is referred to as "shading correction").
However, external leakage light that reaches the second white reference board even when the fluorescent lamp turns off would cause the second white reference board to be incomplete or whitish black. Light black set as a black reference, or recognition of white as black would cause a read image to be more blackish than an actual image, disadvantageously preventing a high-resolution image reading.